In September and early October 1950, the commander of UN forces in Korea, Gen. MacArthur, was faced with a puzzle: would the Communist Chinese intervene in the Korean war, in the face of North Korea's imminent defeat ?
The North Korean army was thoroughly defeated and in no way able to stop UN forces from advancing to the Yalu and unite Korea. Yet, to committ all UN forces to all-front advance to the Manchurian border would expose them to suffer an attack in an overextended position if Mao did attack. Reports from the ground did suggest that Chinese forces had already crossed the Yalu, but the evidence was unconclusive, and he was specifically forbidden air recognissance beyond the river.
"Damn these treacherous Commies" blurted the general. His instincts would tell it was all a ruse, a bluff to keep him away from the border, and there was no intervention ongoing. Then he reminded that communist forces had relied heavily on infiltration and deception during the war with the Japanese and the civil war, so if the avanguard had sent warnings of Chinese forces, there was a serious danger a covert Chinese advance was underway. For all that he wanted a speedy and glorious conclusion to the war, he couldn't risk everything on a gamble.
Then he looked on the map, and a plan started to form in his mind. Korea's geography offered him a good option. The narrow "waist" just southward of the 40th parallel, where the Korean peninsula joined the continent, between Sinanju and Hungnam, was about 100 miles in shape. A defensive position there could be held easily with the forces at his disposal. Concentrating a strategic reserve on that position would only delay the advance to the Yalu by a month or two, if the Chinese were pulling a bluff. If not, he would fall back to the "waist" defensive line and contain them there. Yes, that was the best option. He started planning the necessary orders.
When the Communist Chinese did attack on November 25, a few of the UN forces among the most advanced positions close or on the Yalu border were swept away, but the bulk of the army made an orderly retreat to the Sinanju-Hungnam line (what would be later called the "Mac line") where on-third of the available forces had been kept as a strategic reserve.
When the Communist forces made contact on the line, on December 15, the line bulked in places, but it kept. US air force was able to concentrate its attacks in the relatively narrow strip between the 40th parallel and the Yalu and inflict serious damage on Chinese supplies and reinforcements.
MacArthur asked for permission to bomb Manchuria, which the President was wont to do, but the issue did not made a serious breach of trust between the two men. MacArthur had been right to choose the "waist" defensive option, which he had explained to Truman on their October 15 Wake Island conference, and the President had been right to authorize him. There was a good working relationship between the general and his commander, and American military prestige had been kept.
By the end of January, UN forces had reorganized and started a counteroffensive. Advance was difficult in the mountain ranges of Norethern Korea and only somewhat easier in the narrow strip of costal plan that made the Western coast. By the end of February 1951, although, the Un forces had reached Songchon on the Western coast, were on the outskirts of the Myohyong San in the centwer and had crossed the 40th parallel on the eastern coast. The Un advance seriously threatened Sinuiju and the Yalu border in the west, and the Communist Chinese were hard-pressed to stop the Americans in the plain. They could exploit Korea's terrible geography to stabilize the border in the center and the east, but in the West it was difficult to do so before the Yalu. The Soviets had sternly refused a direct committment (Stalin was having serious second thought about the wisdom of authorizing the attack on South Korea in the first place and the Soviet borser was the most difficult for the Americans to reach). Mao could send substantial reinforcements to the front, but this risked seriously depleting Communist forces in other theaters. Nationalists in Taiwan were smarting for a second round, and areas of the country were not yet fully pacified, like Yunnan and Tibet.
By the end of March, Sinuiju and Gishu, on the mouth of the Yalu were reached by the Americans, and a decision was made. Units in Fukien, Yunnan, and Tibet were sent to the Korean front, and the communist government declared a "great patriotic war to defend Manciuria", claiming that American forces had crossed the Yalu. On April 22, a massive Communist spring offensive was launched. Isolated South Korean forces were overwhelmed, but otherwise, the Un forces made an orderly and profitable withdrawal to their prepared positions on the ChongChon river and their old "waist" line, inflciting horrendous casualties on the Chinese with air bombing, artillery fire, and local infantry counterattacks. By the end of May the exausted Communists had been completely stopped.
Communist's China open and massive intervention into the war had convinced a reluctant UN General Assembly to declare Communist China an aggressor and authorize direct attacks against its territory. US Congress declared an "effective state of war" existed and authorized a major expansion and mobilitation of the US Armed Forces. American forces in Western Europe and on the Korean front were doubled in size. Massive round-the-clock bombings of supply lines, reserves, and industries in Manciuria was started. Soviet protestations were strident, but Stalin was fully aware that USA nuclear potential overwhelimgly exceeded Soviet one, and the Russian manpower reserves were almost wholly exausted by World War II. A second war in Europe would spell disaster, and saving Mao's prestige was not worth it.
The Nationalist army in Taiwan was fully restocked and retrained by the Americans and prepared for an amphibious offensive on the continent. On May, 23 the swelled UN forces started a massive counterattack. By late June, the badly-beaten Communist forces collapsed and a retreat started. By early July the UN forces were back to Sinuju and the Yalu river on the west, the Paitou montain range in the center, and Songjin in the east. On July 23, Jacob Malik, the Soviet United Nations representative, asked for a cease-fire. Truman flatly refused any truce talk that would not involve the Communist Chinese forces withdrawing beyond the Yalu, and the reunification of Korae under the Republic of Korea government. By late August, the UN forces had reached and started consolidating positions on the western course of the Yalu river as east as Chosan (what would be the eastern limit of the Supong Reservoir) and had reached the Kanggye-Kapsan-Kilchu line.
Truman and MacArthur sent warnings through covert diplomatic channels that the UN forces would eventually cross into China if they reached the whole Korean border and a truce that would leave Korea united and safe from Communist threats would not be reached. It was not stated, but it was implied that this would involve an American-backed Nationalist amphibious offensive in Fukien as well. By October, the UN forces had reached and consolidated positions on the whole Yalu river portion of the Korean border, up to Hyesanjin, and were pressing on Chongjin. Truce talks were resumed. The Un command and the USA government were stern that offensive operation would not cease until the whole Korean territory was cleared of Communist presence. Bombings of Manchuria targets were intensified. On November 12, the outskirts of Chongjin were reached and the Communist Chinese command agreed to pull their troops out of Korea. On November 27, 1951 an armistice between the UN and the Chinese Communists was signed. The UN and the USA had won a decisive victory in the Korean War.
In the USA, Truman and MacArthur were hailed as heroes and there was already increasing talk in the Democratic Party and popular pressure for a Truman-Macarthur ticket in the 1952 elections. The course of the Korean War had killed the momentum for the ratification of the XXII Amendment and it was only later that a more moderate version was ratified, putting a limit of three consecutive mandates for any US President. In Europe, the prestige of Communist parties took a serious blow, the popularity of the NATO had a massive upswing and an irresistible momentum for European military-political unification in the European Defense Community (which would integrate West Germany and Western-occupied Austria) started. Talks were started for a combined French-American intervention to "clean up" Vietnam of Communist insurgency.